Dangerously high radiation levels make removing melted fuel and other debris from the plant hit by a huge tsunami in 2011 the most daunting challenge in the decades-long decommissioning project.

Around 880 tonnes of hazardous material remain inside the power station, the site of one of the world's worst nuclear accidents after a tsunami was triggered by a 9.0-magnitude earthquake.

A four-minute video released on Wednesday by plant operator TEPCO showed a snake-like robot arm -- measuring 22 metres and weighing about 4.6 tonnes -- moving through small tunnel-like passages and inspecting complex structures within a confined space.

The robot is equipped with a camera and "is better at retrieving information" than previous devices used, company spokesman Isao Ito told AFP.

TEPCO plans to use the robot later this year to conduct its third trial debris removal operation at one of the melted reactors in the Fukushima Daiichi nuclear power plant, he said.

Tiny samples of radioactive material have twice been collected under a trial project using special tools, but full-fledged extractions are yet to take place.

TEPCO announced in July that the massive operation to remove debris had been delayed until at least 2037. The company previously said it hoped to start in the early 2030s.